This section is intended to provide a background or context to the disclosed embodiments that are recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Watermarks are substantially imperceptible signals embedded into a host content. The host content may be any one of audio, still image, video or any other content that may be stored on a physical medium or transmitted or broadcast from one point to another. Watermarks are designed to carry auxiliary information without substantially affecting fidelity of the host content, or without interfering with normal usage of the host content. For this reason, watermarks are sometimes used to carry out covert communications, where the emphasis is on hiding the very presence of the hidden signals. Other widespread applications of watermarks include prevention of unauthorized usage (e.g., duplication, playing and dissemination) of copyrighted multi-media content, proof of ownership, authentication, tampering detection, content integrity verification, broadcast monitoring, transaction tracking, audience measurement, triggering of secondary activities such as interacting with software programs or hardware components, communicating auxiliary information about the content such as caption text, full title and artist name, or instructions on how to purchase the content, and the like. The above list of applications is not intended to be exhaustive, as many other present and future systems can benefit from co-channel transmission of main and auxiliary information.
In some applications of watermarks, such as copy management applications, the watermarks that are embedded in a host content must often be extracted in consumer products (e.g., media players, copiers, etc.) that access and use the content such as video, audio and images. In applications where the watermark extractors are widely available to the general public, it is reasonable to expect reverse engineering attacks that attempt to understand the operation of watermark extractor (e.g., the watermark extraction algorithm) and to obtain secret information associated with the watermarks (e.g., the stego key). Once the watermark extraction algorithms and stego keys are known, the attacker can manipulate the host content in such a way to, for example, interfere with the extraction of embedded watermarks, manipulate copyright enforcement actions or to insert unauthorized watermarks in the host content. In this disclosure, watermark extraction means the extraction of watermark payload, i.e. the message carried by the watermark, and it doesn't mean the removal or erasure of watermark from the host content. Alternatively, watermark extraction can be called watermark detection, but sometimes watermark detection is used to indicate detection of the presence of a watermark within the host content without actually extracting the payload carried by the watermark.
Some protection techniques such as implementation of code obfuscation measures and tamper resistant implementation of watermark extractors can hamper the attackers' efforts. However, those measures may be costly to implement or may be improperly implemented, thus allowing a tamper-prone device to be released in the market place and be subject to reverse engineering attacks by a large community of resourceful hackers. Moreover, due to the number of different media player and copier types and brands that are currently operating on different platforms, it is likely that a large number of different implementers will be involved in implementation of watermark extractors in their products, which leaves open the possibility that extractor implementation details and secrets can be inadvertently or intentionally leaked to the public.